1. Field of the Invention
The present invention relates to a method for playing back a multisession disc. In particular, the present invention relates to a method for playing back a multisession disc in which music data of a CD-DA format is recorded in at least one session and compressed music data of another format is recorded in another session.
2. Description of the Related Art
One session corresponds to from the start to the end of a writing onto a recordable medium. A disc, such as a CD-R medium, which allows data to be written multiple times, may yield a multisession disc. FIG. 7 illustrates a multisession disc. In the multisession disc, each of added sessions includes a program area P1, P2, or P3, and a lead-in area LI1, LI2, or LI3 and a lead-out area LO1, LO2, or LO3 sandwiching the program area. A volume descriptor (described later) and a path table of an added session and an already written session are recorded in each of the lead-in areas LI1 to LI3. Therefore, by reading the lead-in area of the last session, the file structure of the entire disc volume can be obtained.
Music data of various formats can be recorded in each session of the above-described multisession disc. For example, non-compressed music data can be recorded in a session in the CD-DA format (the same format as in an ordinary compact disc), and compressed music data can be recorded in another session in a CD-ROM mode 1 format. When an enhanced CD is used, CD-DA data is recorded in the first session and CD-ROM data (compressed music data) is recorded in the second session and subsequent sessions. Compressed music data may use MP3, WMA or other compression methods.
FIG. 8 illustrates a CD-DA signal format. The sample data of an acoustic signal has 6 samples of L and R channels (12 samples in total, one sample is 16 bits). The sample data is divided into 24 symbols, each of which having 8 bits. An 8-symbol correction code (CIRC) is added to the 24-symbol sample data so that 32 symbols form one unit. A frame synchronization signal and a sub-code are added to the 32-symbol unit to form one frame ((a) and (b) of FIG. 8). The 8 bit sub-code and each symbol are converted to 14 bits by EFM modulation. The frame synchronization signal has 24 bits. Further, a 3-bit junction bit is inserted between the individual symbols, whereby one frame has 588 bits.
98 frames form a large frame ((a) of FIG. 8). As shown in (c) of FIG. 8, the sub-code includes 8 bits of P, Q, R, S, T, U, V, and W. The sub-codes of the first two frames in the large frame are synchronization signals S0 and S1. Each of the sub-codes P, Q, R, S, T, U, V, and W includes 98 bits. Among them, the sub-code Q includes 2-bit synchronization data S0 and S1, 4-bit control data, 4-bit address data, 72-bit data, and 16-bit CRC data, as shown in FIG. 9.
The control data indicates whether a track is a 2-channel music track without pre-emphasis a 2-channel music track with pre-emphasis, or a normal data track. The track is a music track (CD-DA) if the second bit is 0 and is a data track if the second bit is 1. Thus, by referring to the control data of the sub-code Q in the lead-in area of each session, it can be determined whether the session is a CD-DA format session.
The 72-bit data packet specifies the song number, the elapsed time from the start of the song, the absolute time indicating the absolute current position with respect to the head of a program area with minute/second/frame, and so on. FIG. 9 shows the 72-bit data format in the lead-in area (TOC). (1) MNR is a song number (track number) and MNR=00 in the lead-in area. (2) POINT indicates the song number (track number) when the start of a song is indicated in TOC. (3) MIN/SEC/FRAME is the elapsed time from the start of a song. (4) ZERO indicates that all is 0. (5) PMIN/PSEC/PFRAME is the absolute position time in which the start of a song from the head of the program area is indicated with minute/second/frame, and corresponds to the index of a disc.
FIG. 10 illustrates the TOC in the lead-in area in case of CD-DA. The start of each song is repeatedly (three times) recorded at the MNR=00. For example, the start position of the song 01: 0 minute, 2 seconds, and frame 32, is indicated by the sub-code Q of the large frames Nos. n, n+1, and n+2, and the start position of the song 02: 10 minutes, 15 seconds, and frame 12, is indicated by the sub-code Q of the large frames Nos. n+3, n+4, and n+5. Further, POINT=A0 indicates the first song number in the disc, POINT=A1 indicates the last song number in the disc, and POINT=A2 indicates the head position of the lead-out area. In FIG. 10, the number of songs is 6.
FIG. 11 shows the structure of one sector in the CD-ROM mode 1. Herein, the signal format is the same as in CD-DA. Also, FIG. 12 is a diagram for comparing one sector of the CD-DA mode and one sector of the CD-ROM mode 1.
In the CD-ROM mode 1, the length of a sector is the same as that of the CD-DA mode, that is, 2352 (=24×98) bytes. One sector includes:                (1) synchronization signal data SKD of 12 bytes;        (2) header HDD of 4 bytes;        (3) user data USD of 2048 bytes; and        (4) auxiliary data EDCC of 288 bytes for error correction/detection.Also, the sector includes 98 frames (one frame=24 bytes).        
Among the 4 bytes of the header HDD, 3 bytes (MIN/SEC/SECTOR) corresponds to address information and 1 byte corresponds to mode information indicating the type of data. The address information is represented by minute/second/sector as in the sub-code of CD-DA and basically includes the same elements as in the Q channel. As in the signal format of the CD-DA, a synchronization signal, a sub-code, and an error-correcting code (CIRC) are added to each frame.
When data is recorded in a CD-ROM, data is compiled in one sector unit, a sub-code and a CIRC are added thereto as in the CD-DA, EFM modulation is performed so as to write the data, and the data is read from the CD-ROM in each sector by using the sub-code (absolute address of the Q-channel).
As described above, data in the CD-ROM includes a logical sector of 2048 bytes and includes logical blocks whose number is 2n (n=0 in many cases). A volume space includes a system area and a data area. The system area occupies 16 sectors (logical sectors No. 0 to 15) from the head. In case of ISO9660, the data area includes a volume descriptor, a path table, a directory, and data, as shown in FIG. 13A. The volume descriptor includes a basic volume descriptor, a sub-volume descriptor, a volume block descriptor, etc.
As shown in FIG. 13B, the basic volume descriptor includes a volume identifier A0, a logical block length A1, the size of path table A2 for specifying the file structure (see FIG. 14), the position of the path table A3, and so on.
As shown in FIG. 13C, the path table includes a directory identifier (folder name) B1, the number of parent directory B2, and the record position (extent position) of the directory (file) constituting a directory (folder) B3. A plurality of path tables of a directory, whose parent directory is a root directory, are sequentially recorded.
As shown in FIG. 13D, the directory record includes a file identifier C1, a file start position (extent position) C2, date of file creation C3, the data length of a file C4, etc. The directory records of a plurality of files belonging to the directory (folder) are sequentially recorded.
FIG. 14 is an example of the file structure. The root directory includes a plurality of directories (folders: R&B, ROCK, JAZZ, POPS, etc.), the folder ROCK includes sub-folders ROCKMAN and ROCKWOMAN, the folder R&B includes a plurality of MP3 files (MP3 music files) RB-1, RB-2, etc. Likewise, each of the other folders includes several MP3 music files. This file structure can be obtained by analyzing the above-described basic volume descriptor, path table, and directory.
In CD-DA of the CD-DA format, the number and the position of songs can be recognized more easily than TOC information. Thus, sound can be quickly output when playback is performed. However, when a multisession disc, in which compressed music data of MP3 or the like is recorded in at least one session, is played back, it takes a long time for sound to be outputted. This is because the record structure (file structure) of each session must be analyzed to recognize the folders, file structure, and path structure, prior to commencement of playback. Thus, when compressed music data of MP3 or the like is recorded in the ISO9660 format, substantial time is required to analyze the record structure of the music data. The time for analysis becomes longer as the number of sessions increases. In this case, a delay ranging from several tens of seconds to one minute may be required for commencement of playback in a random playback as well as in a normal playback. Further, a silent state disadvantageously continues during that time.
Also, when a random playback is performed by using a CD changer, the record structure of music data of all sessions of all CDs must be analyzed. Thus, even more time is required for starting playback.